This invention relates to starting systems for AC induction motors, and is more particularly concerned with a solid-state motor start circuit that supplies AC current to the motor auxiliary or start winding on the basis of voltage potential in the auxiliary winding.
At start-up, AC single-phase induction motors require some sort of starting mechanism to rotate the magnetic field of the field windings so as to generate sufficient torque to start the rotor. The starting mechanism enables the rotor to overcome the static forces associated with accelerating the rotor and load, for example, from a zero initial angular velocity.
The typical single-phase induction motor armature is equipped with two sets of windings, namely one or more run windings for driving the motor at normal operating speed, and an auxiliary winding or start winding to generate the required starting torque. In order to provide the necessary rotating magnetic field for start-up, a phase shift device such as a capacitor is connected in series with the start winding. During start-up, both the run winding(s) and the auxiliary or start winding(s) are energized to bring the motor up to a sufficient operating speed. At that point, the start or auxiliary winding drops out of circuit so that the motor operates on the run windings only, for more efficient operation. In the event that a heavy load is encountered and the motor rpm slows significantly below the design operating speed, the auxiliary ,winding cuts back in to increase motor torque to overcome the increased load.
In most AC induction motors, the structure of the auxiliary winding is such that sustained connection to the AC line voltage could cause overheating and damage. For capacitor-type motors, the start capacitor can also sustain damage from sustained connection. Therefore, the start mechanism should both connect and disconnect at the proper times during and after start-up and during and after high load conditions.
One conventional approach to control of the start circuit employs a centrifugal switch. The centrifugal switch is an electromechanical device that is usually located on the armature or shaft of the rotor. Alternatively, an electromechanical current-sensing relay can be used. These devices have limited lifetimes because of arcing and wear problems.
Centrifugal switches are also susceptible to humidity, dust, or corrosives in the atmosphere and typically have a life expectancy far below 1,000,000 operations at full load. Centrifugal switches can pit and then weld closed so that AC current is applied continuously to the start windings. Also, as the device ages, the cut-in and cut-out speeds of the centrifugal switch can vary.
A reed-switch/triac start control circuit is described in U.S. Pat. No. 3,766,457 to Fink et al. There the magnetic field generated by the run windings actuates a reed switch which gates a triac on. This type of switch has a strong positional sensitivity and reed switch noise can cause misoperation.
A solid-state motor start circuit is described in Lewus U.S. Pat. No. 3,916,275. Lewus' approach is to measure the voltage across a current sense resistor in line with the run windings, and if the voltage exceeds a trigger threshold a gate signal is applied to a triac in series with the start winding. Because some power is consumed across the sense resistor, this resistor must be as small a value as possible. However, this necessitates the triac having an extremely high gate sensitivity. The high gate sensitivity makes calibration difficult. Also, the Lewus circuit is strongly sensitive to fluctuations in line voltage.
With this type of circuit it is not possible to stack triacs for higher voltage operation.
Another solid state motor start circuit is described in Kadah U.S. Pat. No. 4,820,964. In that circuit a solid state switch, such as a triac, is used to control the start current, and the switch is gated by a photosensitive element. A light-emitting element bridges across an impedance in series with the run windings to produce an output that varies in proportion with the field current. A light conduit connects the light emitting element to the photosensitive element. This circuit is relatively insensitive to temperature and voltage fluctuations, and is not difficult to calibrate. However, it does require a series impedance in circuit with the run windings.